Scheduler core with period keys and catch-up (Feature 20) (#171)

Co-authored-by: orchestrator-agent <20+orchestrator-agent@code.cubecraftcreations.com>

internal/scheduler/export_test.go

@@ -0,0 +1,9 @@
+package scheduler
+
+import "time"
+
+// SetClock makes now the clock and after the timer s fires by.
+func SetClock(s *Scheduler, now func() time.Time, after func(time.Duration) <-chan time.Time) {
+	s.now = now
+	s.after = after
+}